This invention relates to turbine rotors of gas turbines, steam turbines, etc., and, more particularly, to a turbine rotor comprising a plurality of shafts and turbine discs each with including a blade secured to an outer periphery of a turbine wheel, a spacer mounted between the two adjacent discs, and with the rotor being provided with means for preventing a leakage of air flowing through the interior of the rotor for cooling the through an outer end of the spacer.
Generally, a rotor formed by combining a plurality of discs and shafts is constructed such that discs and shafts are stacked in a superposed relation and secured to one another by bolts so as to form the rotor.
However, a rotor of the aforementioned type suffers the disadvantage that air leaks through the outer end of the spacer in large amount as will be more fully understood from the following description of a cooling air system of a prior art rotor shown in FIG. 1.
More particularly, as shown in FIG. 1, a rotor includes at least first and second stage a wheels 1 and 3, a plurality of blades 4 and 5 respectively secured to outer peripheries of the wheels 1, 3 to form discs generally designated by the reference numerals 1a and 3a, a spacer 2 being interposed between the two discs 1a, 3a and shafts S1 and S2, stacked in superposed relation, secured to the discs 1a, 3a. A combustion gas flowing in a direction incidated by an arrow A impinges on the blades 4 and 5 to rotate the rotor. The wheels 1, 3, in the form of discs coaxial with the shafts S1, S2, are respectively formed with air passages 1b and 3b in central portions thereof. In this case, air 9 for cooling the blades 4 and 5 is led to the interior of the rotor through inlet ports 10 and flows through the air passages 3b in the central portion of the rotor before reaching an inner space 21 of the spacer 2 between the first stage wheel 1 and the second stage wheel 3. The spacer 2 is formed with a plurality of slits 8 at a surface thereof contacting the first stage wheel 1 for maintaining communication between the inner space 21 and an air sump 6 adjacent the outer periphery of the rotor. Thus, the air 9 for cooling the blades 4 and 5 flows through a channel constitutetd by the inlet ports 10, the central portion of the rotor, i.e. air passages 1b and 3b, inner space 21 of the spacer 2, slits 8 and cooling air sump 6.
As shown in FIG. 2, the cooling air 9 entering the cooling air sump 6, after flowing through the slits 8, is introduced into a bottom groove 7 between the first stage wheel 1 and the first stage blade 4 and then flows into radially extending blade cooling ducts 12 in the first stage blade 4 to cool same, before being vented through a top portion 41 of the blade 4.
To enable the cooling air 9 to attain the end of effectively cooling the first stage blade 4, the cooling air 9 only has to be led from the cooling air sump 6 to the bottom groove 7 without any air leaks. To this end, attempts have been made to provide the blade 4 with a projection 42 which is located at a lower portion of the blade 4 and extends toward the center of the rotor in such a manner that it overlaps the outer periphery of the spacer 2 to prevent air leaks through the outward end portion of the spacer 2, as viewed in a peripheral direction of the spacer 2. However it is impossible to bring the first stage blade 4 into intimate contact with the spacer 2 during operation to eliminate a gap 11 between the blade 4 and spacer 2 since it is necessary to provide a clearance between the projection 42 of the first stage blade 4 and the spacer 2 to avoid impinging of the projection 42 against the outer peripher of the spacer 2 which might otherwise occur due to an accumulation of allowed tolerances of these parts when they are separately fabricated. The provision of the clearance is also necessary in view of deformation of the first stage wheel 1 and the spacer 2 which would occur during operation due to centrifugal forces and thermal stresses.
As shown in FIG. 3, length 11' of the gap 11, initially 0.24, becomes longer by 0.05 mm during operation because the first stage wheel 1 undergoes deformation in larger amount than the spacer 2 due to the centrifugal forces of the first stage blade 4. Thus, the gap 11 has a length of 0.29 during operation. In FIG. 3, the line 13 depicts a rise in rpm and a line 14 depicts a rise in a load. As apparent from FIG. 3, the length 11' of the gap 11 increases as the rpm rises and continues to increase even after the rpm has become constant, until the load becomes substantially constant, and thereafer the length 11' becomes flat.
As shown most clearly in FIG. 4, leaks 17, in kilograms per second, are substantially proportional to the length 11', and the leaks are large even if the gap 11 is small since the spacer 2 has a large diameter at its outer periphery and the cooling air 9 in the cooling air sump 6 is a high pressure. The leaks 17 represent about 40% of the air cooling the first stage blade 4 and about 0.5% of the main gas flowing in stream.
In FIG. 5, the ordinate represents a percentage reduction in thermal efficiency and as can be seen from FIG. 5, the thermal efficiency shows a reduction of 0.25% which represents a great loss.
Thus, the prior art suffers the disadvantage that the leaks through the gap between the projection 42 of the first stage blade 4 and the spacer 2 cause a reduction in thermal efficiency. This also gives rise to the problem that the gap 11 shows a change in size due to deformation of the wheel 1, 3 and spacer 2 during rotor operation and the cooling air has nonstatic stability.